1. Field of the Invention
The present invention relates to a magnetic recording/reproducing apparatus such as an audio tape deck employing a magnetic tape as a magnetic recording medium, and more particularly, it relates to a magnetic recording/reproducing apparatus using no capstan for conserving the travelling speed of a magnetic tape.
2. Description of the Background Art
In general, a magnetic recording/reproducing apparatus employing a magnetic tape implements constant-speed travelling of the magnetic tape by holding the tape with a capstan and a pinch roller which is pressed against the same and rotating the capstan at a constant speed.
The roll radius of such a magnetic tape which is taken up on a reel changes with time, whereby the rotational speed of the reel also changes with time when the magnetic tape travels at a constant speed. If a single drive motor is adapted to rotate/drive both of a capstan shaft for driving the magnetic tape at a constant speed and a reel shaft for taking up the magnetic tape as received on the reel, therefore, the reel shaft driving system must include a slip mechanism.
In addition to the function of driving the magnetic tape at a constant speed, a function of rotating the reel at a high speed is also required for fast-forwarding or rewinding the magnetic tape. In order to implement such functions with a single drive motor, therefore, it is necessary to switch the speed reduction factor from the drive motor to the reel system between operations for driving the magnetic tape at constant and high speeds, and hence the mechanism is complicated.
The aforementioned conventional magnetic recording/reproducing apparatus for driving the magnetic tape at a constant speed with the capstan further has the following problems:
In such a magnetic recording/reproducing apparatus, it is necessary to drive the magnetic tape at a constant speed with the so-called wow and flutter value of about 0.3% in recording/reproduction, while driving the magnetic tape at a speed of several to several 10 times that in recording/reproduction in high-speed travelling of the magnetic tape.
In order to enable reciprocation of the magnetic tape with the so-called auto-reverse function, on the other hand, a capstan, a pinch roller and the like are required for forwarding and reversing the magnetic tape respectively, with further requirement for a switching mechanism for reproduction, fast forward and rewind modes. Thus, the conventional magnetic recording/reproducing apparatus is provided with a complicated mechanism which is formed by an extremely large number of components, and inhibited from reduction in weight and cost of the driving system.
A motor rotational speed control technique employed in a conventional magnetic recording/reproducing apparatus or the like is now described with reference to FIG. 48. FIG. 48 is a block diagram showing an exemplary motor rotational speed control circuit for controlling a motor rotational speed with control information from an FG (frequency generator) or a PG (pulse generator) mounted on a motor. The conventional motor rotational speed control circuit shown in FIG. 48 comprises a motor 401 such as that shown in FIG. 49A or 49B, for example, an FG 404 for detecting the rotational speed of the motor 401, an FG waveform shaping circuit 405, an F-V converter 480 for converting speed information to a voltage value, a comparison operator 481 for outputting an amount of error between a target speed and the output of the F-V converter 480, a characteristic setting circuit 482 for setting a gain of the operator output and an amount of phase compensation, and a driving circuit 412 which receives a speed command signal D for driving the motor 401. The FG 404 is waveform-shaped by the FG waveform shaping circuit 408. Thereafter the as-obtained FG signal is inputted in the F-V converter 480, to be converted to a voltage level. This FG level signal is compared with the target value, to form error information. This error information is subjected to setting of a servo gain and an amount of phase compensation by the characteristic setting circuit 482 and forms a speed command signal D, which in turn is inputted in the motor driving circuit 412 for controlling the rotational frequency of the motor 401.
Japanese Patent Laying-Open Nos. 59-165983 (1984) and 2-290175 (1990) describe examples of conventional techniques of correcting/controlling irregular rotation. The technique disclosed in Japanese Patent Laying-Open No. 59-165983 is adapted to access correction data in response to outputs of position and speed detectors which are mounted on a motor, thereby controlling the gain of a speed control system. On the other hand, the technique disclosed in Japanese Patent Laying-Open No. 2-290175 is adapted to extract a set frequency component from irregular rotation components and produce a sine wave which is identical in phase and level to this frequency component for mixing the same to a driving circuit reference signal.
In the aforementioned conventional correction techniques, however, that for controlling the gain of the speed control system by the correction data has such a problem that it is difficult to produce and measure the correction data. In the method of extracting a prescribed frequency component from irregular torque components for producing a sine wave which is identical in phase and amplitude to this component and inputting the same in a motor driving circuit, on the other hand, correction circuits of the same number as components are required in order to correct a plurality of components, while a circuit for synchronizing frequencies is inevitably required when the motor rotational speed fluctuates, although this technique is capable of moving/setting the phase and the amplitude. Thus, the circuit scale as well as the number of adjusted portions may be increased.